1. Field of the Invention
The present invention relates to an LED platform having an LED chip on a membrane.
2. Related Technology
It is known to provide high brightness (HB) LEDs and mid-power LEDs in a so-called package, in which the LEDs are mounted on a supporting substrate.
The LED chip thereby is mounted on a submount comprising some dielectric material that has mechanical properties and relatively good thermal conductivity. The currently used materials for the substrate and the submount require a minimum manufacturing thickness.
For the face-up (FU) LED chip design the substrate of the LED chip can be bonded to the submount using an adhesive or solder.
The present development of flip chip (FC) technology is to minimize the thermal path to the package allowing improved thermal control of the light emitting semiconductor material. For FC application bonding techniques such as gold stud bump bonding can be used that only connects the two components at a low percentage of the LED electrode area. However, this mounting technique is expensive and offers limited mechanical support and limited thermal properties.
The electrical solder connections are ideally on the opposite face to the LED mounting. These interconnections add cost and complexity to the design and can be difficult to achieve in practice in metal or dielectric materials especially at a thin material thickness. Currently alternative methods for making the contacts to the solder face are preferred to the fabrication of electrical feed-throughs.
The LED chip and sub-mount assembly are then mounted into some final package to allow the electrical contacts and light output to be established.
Now, there is the problem that high brightness (HB) LED chips are operated with increasing power. The light conversion efficiency of the LED chip is quite low, thus that the heat generated by the LED chip has to be removed to the ambient through package material. The majority of the costs incurred by packaged LEDs are still dominated by the material costs, which in addition to the LED can be relatively high compared to the overall manufacturing costs.
It is a problem that often the combination of the LED substrate, the submount and the package material represents the weakest link for the heat transfer from the chip to the ambience. Thermally well conducting material such as metal mounts cannot be used alone, as the electrical contacts to the LED have to be electrically isolated. While this is possible, these materials create a decrease in the thermal conductivity that limits the physical size and power of the chips that can be used in this package to low power devices (e.g. Dimensions typically <300 μm and operating power <100 mW). For high power devices (chip dimensions typically >300 μm and operating powers typically >300 mW), the whole metal housing also heats excessively and this can adversely affect the long-term performance of the LED light source, as all these components are typically temperature sensitive.
Where the LED chip does not have uniform dimensions, i.e. no square shape, the operating power is typically the dominant characteristic and the average chip dimension can be used to define the (effective) chip length (L) where the chip area (L×L) is reasonably accurate.
Using thicker dielectric materials with a very high thermal conductivity represents advantages for a submount, but involves high costs and places constraints on the overall device design.
Additionally, the submount must provide for a stable mechanical support for the LED. This requires minimum thickness of the submount. There are conflicting requirements between the ability to conduct the heat away from the LED and the ability to provide a mechanical support for the LED. It is known that reducing the thickness increases the manufacturing costs and makes the use of discrete submounts almost impossible to handle without breaking during the normal LED assembly processes.
At the present time, the design is typically to ensure the mechanical properties of the package at reasonable cost and to accept the then thermal performance that is achieved.